I placed a buy bet on bitcoins with Spreadex today.The round turn spread is about 5%,and the annualised cost of running the position is just over 5% [LIBOR plus 5%].The margin is 50% of the value of the contract.
It is a lot simpler than actually buying bitcoins from ,say Mt GOX,and finding some safe way of storing them.
PLUS 500 also seem to be offering spread betting on Bitcoins.Their spread is narrower,but their annualised cost of running a position seems to be higher.IG offer some binary specials,but they look unattractive.